In presses and dies, it has been common to utilize die cylinders which communicate with a manifold and function as die springs. It has also been suggested that self contained gas springs be provided as shown in U.S. Pat. Nos. 4,742,997 and 4,741,518.
A major consideration in connection with the design of the self contained gas springs is that when charged with gas, such as nitrogen, at high pressures on the order of 1000 p.s.i., the normal stroke of the piston therein substantially increases the pressure. Accordingly, the construction of the self contained die cylinder must be constructed to withstand such high pressures. This results in the need to increase the geometric spaces in which the die cylinders are used. It has thus been customary to compromise the design.
Among the objectives of the present invention are to provide a self contained gas spring which is functional or operation at relatively high pressures; but which does not increase substantially during the movement of the piston within the cylinder which is compact; and which requires a minimum of maintenance.
In accordance with the invention, a self contained gas spring comprising a housing having a closed end and an open end, and a cylindrical bore. A cylindrical sleeve is inserted into the bore of the housing. The sleeve has at least a portion thereof forming an internal cylindrical surface. A piston rod has a piston within the cylindrical surface. The sleeve defines a space between the sleeve and the bore of the housing. This space communicates with the space of said cylindrical surface at the piston end of the piston rod. A charging valve is provided in the closed end of said housing for charging the space at the piston end of said piston rod and the space between the sleeve and the housing with gas under pressure. Preferably, the die cylinder includes sufficient volumetric space such that the pressures in the self contained gas spring do not increase substantially during the movement of the piston within the cylinder.